Tark is a word in the Orcish-Westron pidgin; it refers to a Gondorian human:
In this jargon tark, 'man of Gondor', was a debased form of tarkil, a Quenya word used in Westron for one of Númenorean descent.
Return of the King Appendix F I "The Languages and Peoples of the Third Age" Of Other Races
Snaga assumes, not entirely incorrectly, that only an Elf or a Númenórean would have the power to bypass the Watchers.
Although there's no evidence that they actually have any especial power to do so1, there's ample reason for Snaga to believe them capable:
Númenor/Gondor has historically been closely allied with the Elves, the most magically-advanced race in Middle-earth
The Gondorians are descended from the only armies to ever defeat Sauron: the Númenórean armies of Tar-Minastir, who routed Sauron's forces during his first expansion across Middle-earth; the Gondorian armies of Elendil, from the Last Alliance; and the Gondorian armies of Eärnur, who (admittedly with Elvish assistance) drove the Witch-King out of the North.
We know that modern Orcs have knowledge of the Siege of Barad-dûr, so it's not a stretch to imagine that the descendants of Númenor have built up a reputation in the minds of the armies of Mordor
It's possible that the Watchers were built (at least partially) by Gondorians at the same time as the Tower itself2; if so (or if the Orcs believe that to be so), then it's not unreasonable for Snaga to assume that a Gondorian would know of some weakness in their construction
1 We don't actually know a whole lot about the Watchers at all, so it's not entirely clear by what mechanism (other than Galadriel's Phial, of course) they even could be bypassed
2 Personally, I don't find this likely. But then, it's about what Snaga believes, not what I believe